This project is directed toward the development and construction of an ion-microprobe analyzer which will permit the mapping of stable isotopes, particularly tracers in biological materials, to an accuracy of 100 Angstrom units in some cases, more commonly, 300 Angstrom units. It does this by (1) a liquid gallium field ionization or electrohydrodynamic ion source of a brightness of 10 to the 6th power A ster-1 cm-2, (2) an enhancement of the yield of secondary ions by deposition of cesium, and (3) a secondary ion collection system of high efficiency which puts 10% of the secondary ions into the acceptance phase space of a mass spectrometer with a resolution high enough to resolve 17O and 16O'H. BIBLIOGRAPHIC REFERENCES: "Microscopic Tracing of Deuterium," Walter Kisieleski and Roy Ringo, Jour. of Microscopy 104, 199-204 (1975). "Ion Source of High Brightness Using Liquid Metal," V.E.Krohn and G.R. Ringo, Applied Physics Letters 27, 479-481 (1975).